1. Field of the Invention
The present invention relates to a method and device for reading data in the form of data pulses in a pulse train which contains a sequence of clock pulses which, at least in a portion of the pulse train, define mutually subsequent bit cells which each contain at most one of the data pulses, a bistable circuit being switched to its one state by the clock pulses and switched to its other state by the data pulses.
2. Description of the Prior Art
Such a pulse train is obtained, for example, on the sensing of a magnetic disc, such as "flexible disc" on which data was registered in accordance with the so-called "double frequency" method, that is to say in bit cells defined by registered clock pulses, a registered data pulse in each one of the bit cells representing a logic 1, and the absence of a registered data pulse representing a logic 0. Such a disc is divided into a plurality of sectors which each contain an identification field and a data field. Both of these fields may contain an address section of eight bit cells, an information-carrying section and a control section of sixteen bit cells. For the purposes of synchronization, both the identification field and the data field may be proceeded by 48 empty bit cells, that is to say in which no data pulses have been registered.
For reading data in the pulse train obtained by sensing, for example, a flexible disc on which data was registered in the above-described manner, the contents of the bit cells must be determined, which can be effected by establishing the time interval between the pulses. Since the data pulses are normally registered centrally in the bit cells, it is possible, by means of a monostable flip flop with a pulse duration of, for example, 3/4 of the bit cell time, to determine whether any data pulse is registered in a bit cell or not. If the bit cell time is 4 .mu.s, the pulse time of the monostable flip flop must, thus, amount to between 2 .mu.s and 4 .mu.s.
Even if the registration on the disc is effected with these nominal times, the pulses in the sensed pulse train may be offset in relation to their nominal positions. There are two reasons for this: (a) the pulse intervals vary in proportion to the speed of rotation of the disc on sensing; and (b) the pulse intervals vary with the data pattern. This latter variation depends upon the fact that a high packing density is used for registration on the disc, which entails that, on sensing, the gap of the magnetic head which is used has a not insignificant width, being rather comparable with the distance of the flow changes. Consequently, the magnetic head senses not only the separate flow changes but is also influenced by adjacent flow changes, that is to say the output signal of the magnetic head is dependent upon the data pattern on the disc.
In the information-carrying sections, the influence of the data pattern will be greatest when the bit cells, in which the data pulses are registered, change with bit cells in which no data pulses are registered. If the nominal bit cell time is 4 .mu.s, an "empty" bit cell in the sensed pulse train in this case will obtain a length of 3.2 .mu.s, while the interval from clock pulse to data pulse in the bit cells surrounding the empty bit cell amounts to 2.4 .mu.s. The "window" for distinguishing a 1 and a 0 thereby shrinks from 2 .mu.s to 0.8 .mu.s.
As a consequence of the current appearance of the address section, this window is further reduced, more precisely to 0.4 .mu.s. The reason for this is that, in the address section, which consists of eight bit cells, the sixth bit cell can be empty and those clock pulses which were to have defined the third and fourth bit cells may be omitted. As a result, in the sensed pulse train, the distance between the fifth data pulse and the subsequent clock pulse will be 2.8 .mu.s.
It will be appreciated that a relatively small deviation in speed on sensing further reduces the width of the above-mentioned window and may make it impossible to distinguish between 0's and 1's.
In order to eliminate the reduction of the width of the above-mentioned window by speed variations, phase-locked loops have previously been used, but this is relatively expensive and, moreover, the synchronization is delayed, which makes it more difficult to read consecutive sectors on the disc.